In offshore operations such as oil and gas explorations and productions, installing and protecting cables running from an offshore structure resting on seabed, to other offshore and onshore structures is quite common.
Such an offshore structure may be for example a platform/foundation resting on the seabed for supporting a wind turbine. There can be other examples as known to persons skilled in the art all of which fall within the ambit of the present invention, such as a fixed offshore oil or gas facility or an offshore transformer station. It is also known that for the sake of stability, such foundations are frequently provided with scour protection around the base.
Offshore cables are typically heavy, thick and do not bend in sharp curves. They are also quite expensive, sensitive and costly to repair.
Close to the offshore foundation, the cable needs to be protected from for instance excessive movement, over-bending and damage from dropped objects. Excessive movement is most likely to occur during the installation phase, while the risk for over-bending is often a product of under-scouring of the cable and ensuing free span.
Dropped objects are primarily rocks dumped around the foundation to prevent scouring of the foundation, but also include non-intentionally dropped objects throughout the operational life of the cable. Hence, installation and protection of the cable between the outer edge of the scour protection and point of its entry into the foundation is of vital importance. The cables extend from the seabed to the foundation on the seabed and then to the structure supported by the foundation.
The current state of the art consists of various forms of sheathing or pipes that are installed around the cable, usually at the time of cable installation. The cable is usually installed significantly later than the foundation and its scour protection. Ideally, the cable should go under the scour protection, in which case a costly operation is needed to remove the scour protection to allow the cable and then replace the scour protection to ensure stability of the foundation.
The cable conduits inside the offshore structure will typically be curved (this is always the case for offshore wind foundations). Telescopic arrangements are conventionally thought of as comprising two straight elements, which fit into each other and allow the same shape (straight tube) to be maintained throughout the adjustment of length. Thus, traditional telescopic arrangement is not possible here. A key point of the invention is that the inner tube is elastic, which allows it to assume the shape of the outer tube in retracted position and to return to an essentially straight shape after extension.
Installation and protection of subsea cables extending from, on or into the seabed, to the wind turbine or other structures supported by a foundation, with the help of tubular structures such as J-tubes, are known. Apart from the problem stated in the preceding paragraph, the requirement for divers became almost indispensable for installing such cables into the J-tubes. Even using telescopic J-tubes did not help because divers were required to lock the J-tube in place after installation.
U.S. Pat. No. 7,438,502 teaches a telescopic under-water guiding assembly for subsea elements such as cables. It teaches a telescopic assembly of an outer receiver pipe to which is slidably engaged an inner extension pipe, which can undergo extraction and retraction. A cone end is secured to the inner extension pipe. The latter is locked in a fixed position with binding blocks with set screws.
The above patent does away with the requirement for deploying divers; however it does not teach how the requirement for removal of the scour protection, during its installation can be dispensed with. Further, it does not specifically teach that the protective apparatus can be installed along with the foundation. Furthermore, the arrangement is fairly complicated. These aspects are true for other prior art teachings as well.
US 2010/0196100 shows a tubing arrangement for an offshore facility where a first tube section comprising a curved part is affixed to the outside of an offshore structure and extends from above the sea surface to near the seabed. A second tube part is hinged to the lower end of the first tube part. A third flexible tube part is attached to the outer end of the second tube part. During installation of the structure, the third flexible tube part is coiled up and the second part is pivoted upwardly. When the structure has been installed, the second part is pivoted downward towards the seabed and the flexible tube part is coiled out. The end result is a conduit for a cable or the like to be installed, which extends vertically along the structure and curves into a direction substantially parallel with the seabed.
Although, this arrangement can provide for post-installation of a cable after scour protection has been deployed, it has some major disadvantages. The most important disadvantage is that the tubing is arranged so that it is highly subject to damage during installation. This is especially true for the second and third parts, which extends substantially outward from the structure during installation.
U.S. Pat. No. 7,438,502 shows a straight telescopic tube that is adapted to extend from the deck of a platform to the seabed. The tube can be extended to adjust the length of the tube.
The tube of U.S. Pat. No. 7,438,502 is not suitable for conducting a cable or the like below a scour protection. The tube will form a straight line set at an angle from the deck to the seabed and at best extend above any scour protection. Hence, it permanently installed, the tube will be subject to potential damage from any ships that come too close to the structure. It will also be subject to damage from waves and currents.
U.S. Pat. No. 3,724,224 shows a J-tube that may be pre-installed within the seabed structure. A pipe can be pulled through the J-tube. This pipe is fed from the platform deck after the installation and is pulled along the seabed to another platform. The pipe is exposed between the first and the second platforms. Hence, the pipe has to be installed before any scour protection is deployed around the first platform. If the pipe has to be replaced, the scour protection covering the pipe has to be removed.
U.S. Pat. No. 4,523,877 shows an arrangement similar to U.S. Pat. No. 3,724,224. Here a J-tube is preinstalled within a structure. After the installation of the structure, a riser is fed through the J-tube. The part of the riser that extends through any scour protection is exposed, so that it is not possible to deploy scour protection before the riser is installed or remove the riser without first removing the scour protection.
FR 2378227 shows a protective cover comprising a plurality of shields that can be stacked inside each other during installation. After the installation, the shields are deployed in an overlapping line to form a protective cover for a pipe or cable. However, this system of shields is complicated to deploy and necessitates the use of a diver. Moreover, it is not possible to feed a pipe or cable from a position above the sea surface to a position outside the cover.
Hence, there has been a need for a simple and efficient apparatus to act as a conduit for installation and protection of subsea cables, which dispenses of the requirement for removal of the scour protection of the foundation, during its installation.
There is also a need for a technology that provides such an apparatus, which can be installed along with the foundation, prior to providing the scour protection. This is to ensure that the apparatus extends for a pre-determined length along the seabed, through the zone in which scour protection is to be applied later, so that the cable can be installed through the scour protection since applied, without the need for removing it.
The present invention meets the above mentioned needs and other associated needs by providing an apparatus which has a first tubular member arranged to move telescopically with respect to the second tubular member, such that both can be installed together with the foundation, through the zone of possible scour protection, before it is applied.